Josh Powell claimed he left his wife at home as he took his sons, then ages 4 and 2, on a late-night camping trip in the west desert of Utah. He denied having anything to do with his wife's disappearance.
Josh Powell killed himself and the couple's sons last year when he set fire to their rented home in Washington.
Deputy Chief Mike Powell (no relation to Josh Powell) said Monday that the investigation involved talking to hundreds of people and following up on more than 800 tips. Some of the newly-released documents were sealed to "protect the integrity of the investigation." However, Mike Powell said Monday that "successful prosecution was not likely to happen at this point."
"We have left no stone unturned during the last 3 1/2 years of this investigation," he added.
Mike Powell said during the news conference that investigators believed "early on" that Josh Powell was involved in his wife's disappearance. Josh was the last person to see Susan alive and showed a lack of concern for her disappearance, something Mike Powell described Monday as "frustrating." Investigators also found droplets of Susan's blood in her home, though Mike Powell added that the droplets were not necessarily suspicious.
Investigators and a deputy district attorney subsequently treated the case as a homicide and kidnapping. Mike Powell said Josh also may have been obstructing justice.
Among other things, investigators obtained a wiretap order, which they used to monitor communication between Josh Powell and his 30-year-old brother Michael. The wiretapping began in August 2011 and continued until October. Police also used dogs, underwater search tools, aircraft and other means to look for Susan.
Mike Powell said during the news conference that after Josh's death investigators turned their attention to Michael Powell.
Investigators believed Michael may have transported Susan's body shortly after her disappearance. However, DNA found in the trunk of Michael's car did not match Susan's.
Michael and Josh also used sophisticated computer encryption to communicate, according to West Valley City Deputy Chief Phil Quinlan. Investigators have been unable to decipher that encrypted communication, Quinlan added.
Michael Powell, an ardent supporter of Josh Powell, committed suicide in February of this year by leaping from a building in Minneapolis, where he lived. That left investigators without any person of interest in the case.
At the time of his death, Michael Powell was embroiled in a legal battle in U.S. District Court for Western Washington with Susan's parents Chuck and Judy Cox over $1.5 million in insurance policies issued to Josh Powell and sons Charlie, 7, and Braden, 5.
About four months before his death, Josh Powell changed his policy to list Michael Powell, rather than his wife Susan and a family trust, as the primary beneficiary. Michael Powell was to receive a 93 percent share, with 4 percent left to Alina Powell and 3 percent to brother John. In the event of Michael's death, the proceeds were to be split equally between Alina Powell and their father. Josh Powell also made Michael Powell the secondary beneficiary on his sons' policies.
Police considered that suspicious, Mike Powell said.
Mike Powell also said Monday that the brothers' father, Steve Powell, was not directly involved in Susan's disappearance but may have learned of it afterward. However, he has not been forthcoming with investigators.
"He is entirely uncooperative and wants nothing to do with law enforcement," Mike Powell said.
Steve Powell currently is serving a prison sentence for a conviction on voyeurism charges. He could be eligible for release later this month.
The search for Susan Powell most recently took investigators to a home in rural Scotts Mills, Ore. But after the search was called off after two days when nothing was found.